Integrated optical circuits (“IOCs”) use optical waveguides on solid substrates to conduct and process optical signals. IOCs are used for a variety of applications, including in fiber optic gyroscopes (“FOGs”). A FOG uses light interference to detect the rotational motion of a body. FOGs are highly reliable and can provide precise rotational rate information. IOCs for use in FOGs consist primarily of an optical waveguide Y-junction and an electro-optical phase modulator, and are often constructed using lithium niobate single crystal substrates with Annealed Proton Exchange (“APE”) waveguides. The APE waveguides are highly polarizing, which eliminates the need for a separate polarizer component in the FOG architecture.
FOGs are especially attractive for satellite pointing applications because of the low noise levels that can be achieved. However, performance degradation in orbit, which can be attributed to vacuum exposure of the APE lithium niobate integrated optical circuits, has been experienced in some early systems. Vacuum exposure can increase the electrical conductivity of the APE lithium niobate surface, which non-linearly decreases the electro-optic phase modulator efficiency and introduces noise into the FOG rate measurement.
Previous attempts to provide IOCs which are useful in a vacuum environment have not been successful. For example, U.S. Patent Application No. 2010/0135610 to Honeywell International Inc. addressed the issue of vacuum exposure of the APE by using a vacuum insensitive titanium in-diffused waveguide for the electro-optic phase modulator function, while retaining sufficient APE waveguide optical path length on a separate chip to provide the needed polarization extinction ratio. However, due to the number of fiber-to-chip pigtail connections used, the overall optical loss of this stitched system is increased, which decreases gyroscope performance. In addition, U.S. Patent Application No. 2007/0116421 to Honeywell International Inc. addressed the same problem by using a reactive gas protective coating on the vacuum sensitive surface of the IOC. However, this reactive gas coating loses effectiveness at the elevated temperatures needed for some important applications, including satellite applications.